Bah!
Samwise is a cheat! How can you ask us to bring out quotes from the books? We all know that after Frodo departed, Samwise edited everything to present himself in a better light. He is, in actuality, a bumbling fool. And he is a fat hobbit, as well. How on earth does he stay fat, while Frodo withers away? He ate all the food!
Bilbo is like Willy Wonka. Granted, I love Gene Wilder as Willy, but he's creepy. And so is Bilbo. And the whole butter over bread thing is just code for "I'm tired of this, time to pass on the burden to Frodo". And, he helped kill Smaug, my favorite character from The Hobbit.
Frodo is a tragic hero. That instantly garners disrespect, because he couldn't be wise enough to, oh let's say, give the Ring to Boromir.
Merry and Pippin both do horrible things. Merry helped out in the defeat of Wiki (he should have been burned at the stake for such a crime, seriously). Pippin is just...Pippin.
Thus, I will vote for...Pippin. He neither killed Smaug (well, Bilbo helped), nor Wiki, nor Sauron.